There is a known rolling control structure which is disclosed in Japanese Pat. No. 216603/1983. In this structure, the angle of inclination in the lateral direction of a tractor with respect to the ground surface is detected by a sensor attached thereto, and another sensor is fixed to an extensibly-formed lift rod cylinder provided around one of the left and right lift rods by which the lift arms and lower links of the tractor are connected together, to detect the length of the lift rod cylinder, on the basis of which the angle of inclination in the lateral direction of the ground treating machine with respect to the tractor is calculated. The angle of inclination in the lateral direction of the ground treating machine with respect to the ground surface is calculated on the basis of the angle of inclination in the lateral direction of the tractor with respect to the ground surface and the angle of inclination in the lateral direction of the ground treating machine with respect to the tractor to thereby control the extension and retraction of the lift rod cylinder so that the angle of inclination in the lateral direction of the ground treating machine is in conformity with the posture, which is preset by the operator, in the lateral direction of the same machine.
Since both of the above two sensors are attached to the tractor, the sensor re-attaching operation is not required even when the ground treating machine is changed. However, the correction calculations made when the above angle of inclination in the lateral direction of the ground treating machine on the basis of the values detected by the sensors are very complicated. Therefore, this rolling control structure has a low practicality.
The angle of inclination with respect to the ground surface of the lift rod cylinder in side elevation thereof varies greatly with the vertical movements of the lift arms and lower links. Accordingly, the vertical component of the amount of extension and retraction of the lift cylinder, which is an important numerical value for the calculation of the angle of inclination in the lateral direction of the ground treating machine, varies greatly with the vertical movements of the lift arms and lower links. Hence, in order to determine the vertical component mentioned above, on the basis of the values detected by the sensor, by which the amount of extension and retraction is detected, making complicated geometrical correction calculations is required. In this known structure, it is necessary to detect the angles of pivotal movements of the lift arms by independent lift arm sensors, and correct the heights of the lift arms of the ground treating machine when calculating the angle of inclination in the lateral direction thereof and add these corrected values in the calculation of this angle of inclination. In order to carry out a rolling control operation, it is necessary to make such various kinds of complicated calculations of correction amounts. Moreover, it is difficult to calculate correction amounts accurately in a short period of time.